An automatic testing machine (ATM) operates in a production environment to rapidly and accurately test the operation and performance of various types of devices under test (DUT), including RF communication devices. The DUTs could be a finished product or a component of a larger system.
The ATM is programmed to perform various tests on the DUT automatically. For example, a microcomputer chip DUT may be fed power and known input signals, and the output signal of the DUT compared with expected results. Another example is where RF signals are transmitted to a finished cellular telephone DUT to determine if the telephone properly operates. Other tests could include environmental tests, such as temperature or vibration tests.
Depending upon the nature and number of the tests being performed, the testing may last from a couple of milliseconds to several minutes. The information from the testing is compared with expected test results. If there is some defect so that the DUT falls below specifications, the ATM will designate the DUT as failed, either by marking the DUT, placing the DUT in a failure area, or indicating the failure to an operator.
The ATM is then loaded with the next DUT, either manually or automatically, and the testing procedure is repeated for this DUT. This testing information can be used to evaluate the fabrication process for possible changes, as well as to perform failure analysis on individual failed devices.
ATMs are used perform operational tests on completed products. For example, ATMs will test the operation of a completed cellular telephone. This includes testing the user interface features, such as buttons, slides, switches or levers. The ATM will activate the various interface features on the product and determine whether the product responds accordingly. In order for the ATM to operate the interface features on the product, the ATM must have engagement mechanisms which couple to the interface features and will move the features in their intended manner. For example, if the interface feature is a button, then the engagement mechanism could be shaft with a nib on a distal end which contacts and pushes the button.
ATMs typically use pneumatic air to operate the fixtures which load, clamp, and then unload the devices during testing. ATMs also use pneumatic air to power the engagement mechanism. FIG. 3 depicts an internal view button engagement mechanism 30 which includes actuating shaft 31 having an actuating plunger 32. Plunger 32 is connected to a pneumatic air source (not shown). The other end of shaft 31 is nib 33 which engages the buttons of DUT (not shown) such as a cellular telephone. When pneumatic air is activated, shaft 31 moves and nib 33 contacts the button (not shown). When the air is shut off, the shaft 33 will not return back to its original position, unless acted upon by another force.
The force used to return shaft 33 to its original position is provided by spring 34. Thus, as the shaft 33 moves to contact the button via nib 33, spring 34 is compressed between collar 35 and the side of the housing of mechanism 30. The force of the air overcomes the force of spring 34, and nib 33 contacts the button. When the air is shut off, the spring returns the shaft to its original position. To ensure smooth motion of shaft 31, bearings 36 are used.
However, the use of spring 34 causes problems in measuring the operation of the DUT. The force provided by a spring is not constant. The force varies linearly depending upon the distance of compression. Thus, as spring 34 is compressed, the force generated by the compression will vary. Similarly, when spring 34 expands, the generated force will vary. Therefore, even though the force provided by pneumatic air can be made controllably constant via an air cylinder with a proportional regulator, the resulting force acting on the button will vary because of spring 34. Note that the variable force will occur in both directions, i.e. as the button is being pushed in and as the button is being released.
The variable force causes problems in measuring the performance of the product. The calculations required to determine the precise amount of force being applied to the button are complex, as the amount of force depends upon the stroke of the actuating shaft. Furthermore, springs have compression points whereby the force becomes non-linear with respect to the compressed distance. Moreover, the variable nature of the spring itself is subject to change over time, as springs are subject to wear and elastic breakdown. In a production environment, with thousands of actuating cycles being per performed per day breakdown can occur quickly. Also, no two springs perform exactly alike, as each spring will have different characteristics because of differences in materials and fabrication. Thus, the amount of force being applied to a button is inaccurate, variable, and difficult to determine.
Consequently, collecting data on the performance aspects of buttons is difficult. A constant and known force is needed to determined the activation characteristics of the button, as well as to determine the expected life time of the button.
Therefore, there is a need in the art for a system and method that allows an ATM to exert a constant and known force onto the interface features of devices, particularly button keys of cellular telephones, in a production environment.